Television transmitting system



lNvEN-roR 07'7'0 H. SCHADE alalslnlslllsl o. HL

TELEVISION TRANSMITTING SYSTEM Filed ATTORNEY Patented July 16, `1946 otto n. schade, West Caldwell, N. VJ., assigner to Radio Corporation of America, a corporation of Delaware Application'J une 27, 1941, Srial'NO; 399,995l

My invention relates to television transmitting systems and particularly to systems'utilizing low velocity electron beam scanning tubes.

Tubes of the low velocity electron beam scanning type such as disclosed by Albert Rose in his U. S. Patent 2,213,174 and referred to as Orthicon tubes and wherein an insulated mosaic target surface is scanned by a beam oflow velocity electrons give a signal outputwhich-is proportional to the brightness of an opticalimage focused upon the mosaic target. This output being proportional to the image brightness is due to the fact that during normal operation of the tube an electron collecting electrode which is in close proximity to the mosaic target is maintained at a positive potential with respect to the cathode, while the mosaic surface in the absence of light is at cathode potential, resulting in a strong electron collecting eld preventing the formation of charge eiects which Would limit the photo-emission and produce a nonlinear output. The electrostatic charges developed on the mosaic are thus limited only by the amount of light incident: thereon, and inasmuch as the electron beam ap" proaches the mosaic in the absence of light with a velocity approaching zero any positive charging of the mosaic in response to Ilight tends to accellerate the beam. If such charges are of too great an intensity, such as due to highlights 'or rapid increases in the amount of light, secondary electrons are developed on the mosaic because of the increased beam velocity which are likewise collected by the collecting electrode. This action is cumulative, and as a result, the electron beam is no longer able to discharge the mosaic to develop television signals. In addition, if this action is initiated over a small area of the mosaic target,

the highly charged area spreads until the entire mosaic becomes inoperative. Thus in Vtelevizing indoor sporting events photographers flashlights often cause excessive chargingV of the mosaic target which is only neutralized by readjustment of the various potentials and only after a successive number of scannings by the electron beam resulting in loss of the picture often at times when the action is most desired. This loss of the picture may vary over a time period of from one to several seconds, inasmuch as the tube must be restored to operation'by manual adjustment such asby masking of the optical image and reducing the high electrostatic charge or charges on the mosaic to a datum level.

It is an object of my invention to provide a television transmitting system incorporating a low velocity electron beam scanning tube wherein 9 Claims. (Cl. 178-7.2)

the signal output of the tube may be reduced during periods of high intensity light level. It is another object to provide a system wherein the tube may be made inoperative in response,to excess illumination-and immediately returned to operation without excess delay. It is a further Y object'to provide la system incorporating a 10W velocity g electron beam scanningY tube wherein excess-charging of the scanned target maybe prevented, andiitv is a still further object to'provide means whereby such excess charging is limitedv tov anl amount insulcient to interrupt-the operation of the system for indeterminate periods of time. It is likewise a further object to provide a system wherein excess charging of a scanned mosaic target under excessive highlightsis limited and wherein the slight excess charging of the 'target following such limiting is neutralized.

No special precautions need to be takeny in tubes utilizing high velocity electron beams suchV as of thev Iconoscopeltype in order to attain these objections; and my invention is not relevant to'such tubes or systems utilizing such tubes, because the voltage difference between'the mosaic and the collecting electrode is, at most, a few volts, so that the maximum voltage which the mosaic can reach by emitting photo-electrons is limited. Further, the lighted areas collect in'-A creasing numbers of the redistributed secondary electrons produced by the high velocity electron' scanning beam, as the mosaic becomes more positive. the mosaic.

In accordance withmy invention I provide a:

television transmitting system incorporatingr a tube having a charge storage type target scanned with va low velocity electron beam and means to collect electron emission from theY target with means to interrupt the collection of photo-electrons and prevent or limit Vthe development ofY secondary electrons, such means being responsiveV to the source of image light from the object area which is imaged upon the mosaic target. Further in accordance with my invention Iprovide a system wherein any slight abnormal charging of the targetimmediately preceding the interruption of photo-electron collection `is neutralized. I'hese and otherfobjects', features and advantages of my invention will become apparent in view of the following? description and the accompanying drawing wherein the single figure shows the preferred embodiment of my invention.

Referring tothe drawing, I have shown only one particular type of low velocity electron beam scanning-tube to which my system is particularly This alsov reduces the voltage attained by 3 adapted, the tube I comprising an evacuated envelope enclosing at one end a target or mosaic electrode 2 adapted t0 have formed thereon an optical image of an object represented by the arrow 3 such as through a lens system 4. At the opposite end of the tube I I provide an electron source or cathode 5 adapted to liberate electrons in the form of a concentrated electron beam. The intensity of the electron flow is controlled `by an apertured electrode or grid 6, the electrons being accelerated by an anode I and formed intoa beam directed toward the mosaic electrode 2. The cathode 5 is heated to an electron emissive temperature by a heater connected to a suitable source and the apertured grid 6 is connected through a resistor 8 and to a conventional source of grid biasing potential and to the negative terminal of a potential source such as a battery 9. The function of the resistor 8 in series with the grid 6 will be described in more detail below. The .anode I is maintained at a positive potential with respect to the cathode by the battery 9 such as through the ground connections as shown in the drawing. The mosaic electrode 2 which faces the electron source or cathode 5 may be of the conventional type and comprises a substantially transparent sheet of insulation such as the mica sheet Ill having on its rear surface a translucent or semi-transparent electrically conducting signal plate II, the opposite surface of the sheet of mica being provided with an exceedingly great number of mutually separated photosensitive particle'sl I2. Manufacturing processes for constructing such electrodes are well known, various methods being described by S. F. Essig U. S. Patents 2,020,305 and 2,065,570 and by I-Iickok U. S. Patent 2,178,232. The signal plate I I of the mosaic electrode assembly is connected to the input circuit of a translating device such as a thermionic amplifier I3 and to a potential on the potential source or battery 9 approximating that of the cathode 5 through an output impedance I4. The electron beam, is scanned in one coordinate direction over the mosaic electrode 2 by a pair of deection plates I6, the plates being connected to a source of deflection potential and to ground through a center-tapped resistor of from one to ten megohms. The electrostatic deflection plates IS are preferably immersed in a longitudinal magnetic field such as developed by the coaxial coil Il, the other coordinate of deflection vbeing obtained by the use of a second set of plates or by the magnetic coils I8. To provide a substantially field-free space and uniform velocity of the electron beam during deflection I provide an electrically conductive coating I3 over the inner wall of the tube I extending between the anode and a point in proximity to the mosaic electrode 1 and to insure efficient collection of photo-electrons liberated by the mosaic in response to light I provide an electron collecting electrode 2D which is electrically connected to the wall coating electrode I9, the electrodes I9 and 20 being normally maintained at ground potential and highly positive (D-250 volts) with respect to the cathode 5 and signal plate I I.

In the operation of the tube described and shown in the drawing the electron beam approaches the mosaic with relatively high velocity and is decelerated immediately prior to reaching the mosaic and, in the absence of light suchv as an optical image on the mosaic, fails to reach the mosaic but returns and is collected by the electro'des I9 and 20. Thus while I have described only one type of tube to which my invention is applicable, it will be appreciated that its application is directed to tubes utilizing such low Velocity electron beam scanning. Thus various modiiications may be made in the particular tube structure with which my invention may be practiced, such as by providing the collecting electrode adjacent the electron source or replacing the anode 1, in which event use of full magnetic deection of the beamV is desirabley the only prerequisite being that the mosaic electrode is `scanned by such a low velocity electron beam.

In accordance with my invention, I provide means to prevent collection of electrons such as by the electrodes I9 and 20 during periods when the mosaic electrode is subjected to intense illumination which would tend to cause elemental areas thereof to become charged `to an extent sufficient to materially influence the scanning beam velocity. In accordance with this teaching of my invention I provide a light responsive device, which is subjected to a portion of the light which is used to form the image to be transmitted, to derive a signal responsive to sudden changes in the image light which I use to control the operation of the low velocity electron beam scanning tube.

Referring t0 the drawing, I provide a light responsive device, shown in cross-section at 30, which is preferably of the photo-electron emissive multiplier type having a photocathode 3l, a series of secondary electron emissive electrodes 32, a series of electron directing electrodes 33 and an electron collector 34. I so position the device 30 that the cathode 3l is subjected, such as through a. lens system 35, to the source of light such as the arrow 3 representative of the optical image to be televized. It is imperative for the proper operation of my invention that the light responsive device 30 is subjected to the same object constituents or light sources to which the mosaic electrode is subjected. The lens systems 4 and 35 may therefore be mounted in close proximity and designed to have the same angle of view( It is not necessary, however, that the mosaic electrode 2 and photocathode 3l be subjected to the same intensity of light. I prefer to use a light responsive device which has substantially no time lag or is limited at most in time of response to electron transit time within a single envelope in order to simplify the remaining circuit considerations. Thus it is very desirable to provide a device and associated circuit which is substantially instantaneous in operation or at least has a very rapid response such as at most one to two microseconds. The device 30 may therefore comprise to advantage a tube of the electrostatic multiplier type such as the R'CA type 931. As shown by the drawing, the device 30 is energized by a high potential source 4E! which may be a source of potential of the order of 1000 volts, the cathode 3| being connected to the negative terminal and the collector 34 to the positive terminal through an output impedance comprising the resistor 4I and reactor 42, the secondary emitting electrodes 32 and directing electrodes 33 being maintained progressively more positive in the direction from the cathode tc the collector.

Inasmuch as the device 30 is of the type develcpi-ng an increase Aof current with increase of light intensity, thervoltage drop appearing across ll-42 will be a negative pulse. Since it is necessary to produce a final negative pulse; and the output of the device 30 under normal conditions is insuicient, it is desirable to use two phase reversal and amplifying devices between the device V313 and' theelectrodes I9 and .20 'of the tubegsl; Therefore I connect the collector 34 ofthe device 3U through a condenser 43 to the inputcircuit of a thermionic amplifying device 44 suchyastothe .grid electrode 45. 'I'he device 44 maybe ofgthe triode type including only a single grid between the cathode 45 and anode 41, but I have shown a conventional tetrode having an auxiliary grid 48 between the control grid and anode,. a suitable type being the conventionalrGL. The device 44 produces a positive voltage impulse in response to high light intensities incident on vthe photocathode 3| of the device 30,buttoprovide a negative impulse I provide a second therinionic ainplifying device 44 which may likewise be of vthe 6L6 type having a cathode 46', control grid 4,5, auxiliary grid 48 and anode 4 'I the inputciruit of which is energized from the vanode'l llhrough av condenser 5l). The output of the device 44' is utilized in accordance with my invention to prevent collection of electrons liberated bythe mosaic electrode 2 during periods when excessive light is incident on the mosaic, The electrodes I S and Y2l) are maintained at a normaloperatincr potential such as 250 volts positive with respect to the cathf ode 5 of the tube I Ibya connection to the potential source or battery 9 through ca resistor 5I of predetermined value. the anode 4'I through the condenser 50 modies this potential, reducing it to a zeroY or to a negative value. *i

As mentioned above, it is very desirable.V that the response to highlight Values be substantially A negative impulse fromY vfew frames atV-thefmost. i

yIn accordance with further,.teachingHot-iny invention, 1 I neutralize any residual ,charge :ape pearing ongthe mosaic dueitothe.-lapsefoanite time such as the one to two,.micro seconds,between the initiation of the4 excessive light condition: andl the operation oftheaboveedescribed,,circuitby Y increasing the electron beam intensity; foi-....sevf era 1 f rame scannngs-immediately subsequentto ordurng thefresumption of Voperationrolthe tube' Referring, again'to kthe.d rawing, I have l shown a device 60 suchf asa thermionictampli:

instantaneous and it is likewise adesirable that normal changes in the brilliance .of thelobject of s which an image is to be transmitted producelittle or no response in the control circuit.V YThe device 44 may therefore be biased such as by the battery 52 to respond only to excess values of illumination, which is to say, to excess valuesrofrpotential appearing across the resistor 4I and impedan-ce 42. The response of the circuit to sudden or abnormal changes in illumination maybe'proyided by loading the anode 41 through a4 reactor 5i and resistor 54 of relatively low Value and by vloading the anode 41 through a reactorV 53 and resistor 54 likewise of relatively low value.

In operation, light such as represented by the arrow 3 is simultaneously incident upon both vthe mosaic electrode 2 and the photocathode 3L of the'V device 3U'. The light incident o n Ythe mosaic electrode 2 produces relativelygradual `emission of' photo-electrons, the len's'sy'stem'4 being ad-` justed so that with normal lighting conditions the mosaic may be discharged following a v frame time of 1/3'0 sec. The instantaneous signal response of the device 3U, howeven'land its associated circuit is quite rapid so that following the initiation of an excessamount of light'such'as a photographers flashlamp a negative'voltage surge is available at the output of the device Y44' to be applied to the collecting electrodes I9 `and 26. Little, if any, charging of the mosaic ,has occurred during this period. Thus for a onemicrosecond response of the circuit including the devices 30, 44 and 44 the mosaic electrode twill have charged due to normal light intensity only 30/mnoimu of its normal amount and the Lcharge due to excess illumination'will beinsuicient to disturb the operating characteristics of thetube I following return to normal light intensity. However, for the duration *ofl abnormally highV values of illumination of the mosaic electrode 2 and device 30 the zero or negative potentialwith respect to the cathode 5 applie'dfto .the electrodes fier or vinverter having an input electrode, or,A grid 6I capacitively coupled to the outputV4 circuit of the. device 44 anda plate oranodeeGZgcapaci.- tively coupled through a condenser atothegrid electrode of thetube I. The; outputharactere istie ofthedevice is modiedrgsuchgas byia condenser 64: shu nting the;load resistor $5A to 10b: l

H.Thusthe impulse tain a trailing characteristic applied to the grid lII b'eingnegative produces;a positive impulse in the plate circuit, which -fbeing applied to the;grid6 on,the-gridfside of the re.; sistor. B momentarily increases'the beam current effective to discharge .themosaic. w during theinitial periodofthis impulse isunable to. reach the mosaic because. of V.the negative voltage impulse applied to the electrodes p ulse and due' tothe trailing .characteristicppr vided` byvzithe fcondenser ,resistorycombination Gli-*65, the effective -beamrcurrent .is` increased duringpne. 01` :severalfiramey scansionsfofi-.lieVVV beam immediately-following the end ofrtheim-Y 5 circuits for which my invention maybe employed, Y it will lbe apparentthaty-my Vinvention isv by vno means limited to: the exact formsv illustrated. fo'r the `use indicatecllout that many variations-:may

bemade in ,the tube with which, my invention: is i useful and the purpose for which inyinvention is employedrwithout departing .from-.the scope thereof as setvforth in the appendedl claims. Iclairn: 1.- In a'television transmitting system includinga tube of the lowvvelocity'electron beam scanning type said tube having a. photo-emissive electrode, an electronY collecting :electrode and means'to develop and'scan saidemissive -electrode with l--anf electronbeam having a fve'locity approaching zero, adjacent# said Aemissiveifelectrode, lmeans to'subject said v'emissive electrode to a source of light toliberate electrons normally collectedrby said collecting electrode, an. elecv tron discharge device exposed to and. responsive .'Howeverf, the, beam i "aio-1,09%

to abnormal intensities of said source of light to derive a negative control voltage and means to apply said control voltage to 'said collecting electrode to interrupt the normal collection of electrons by said collecting electrode.

2. In a television transmitting system including a tube of the low velocity electron beam scanning type said tube having a photo-emissive electrode, means exposed to said emissive electrode to develop an electron beam having a substantially zero longitudinal velocity adjacent said emissive electrode, an electron collecting -electrode between said means and said photo-emissive electrode, and means to scan said emissive electrode with said electron beam, means to focus an optical image of an illuminated object subjected to abnormally excessive periodic illumination on 'said emissive electrodeV to liberate electrons normally collected by said collecting electrode and develop an electrostatic image capable of being neutralized by said beam, a phototube exposed to and responsive to abnormally excessive intensities of illumination of said object' to derive a control voltage, means to amplify said control voltage and derive a negative control voltage, and means connected between said phototube and said collecting electrode to apply said negative control voltage to said collecting electrode to interrupt the normal collection of electrons by said collecting electrode and prevent the formation of an electrostatic image incapable ofneutralization by said beam. v

3. In a television transmitting system including a tube of the low velocity electron beam scanning type said tube having a photo-emissive mosaic electrode, an electron collecting electrode adjacent said mosaic electrode to collect electrons liberated from said mosaic electrode in response to light, and means to develop an electron beam having a velocity approaching Zero adjacent said emissive electrode `to scan said emissive electrode, means to form an optical image or an illuminated object subjected to periodic excesses of illumination on said emissive electrode to liberate electrons normally collected by s aid collecting electrode, a secondary electron multiplying phototube exposed to the light forming said image t derive a control impulse representative of the periodic excesses of intensity of said image, and means substantially instantaneously responsive to said derived control impulse connected between said phototube and said collecting electrode to apply said impulse to said collecting electrode to interrupt the normal collection of electrons by said collecting electrode. Y

4. In a television transmitting systemincluding a'tube having a photo-emissive electrode, an electron collecting electrode to collect photoelectrons from said emissive electrode, and an electron beam source, means to normally maintain said collecting electrode at a positive potential with respect to said electron beam source to form and direct an electron beam toward said emissive electrode, means to maintain said photoemissive electrode and said electron beam source at substantially the same potential Vto decelerate said beam in the absence of light on said emissive electrode to substantially zero velocity adjacent said emissive electrode, means to subject said emissive electrode to a source of light to develop television signals, means including afphototube responsive to abnormal intensities of light from said source to develop a negative voltage substantially simultaneously with the Voccurrence of the abnormal intensities from said source and means comprisingan electrical connection between said last-mentionedn means i and said collecting electrode to apply said voltage-to said collecting electrode tov interrupt 'the passage of said beam to said emissive electrodeand the collection of photo-electrons from said emissive electrode.

5. In a television transmitting system including aA tube havingna photo-emissive electrode, an electron collecting electrode to collect photo-electrons from said emissive electrode, an electron beam source, and anrelectron beam control electrode, means to apply to and normally'maintain said collecting electrode at a positive potential with respect to said electron beam source to form and direct an electron beam toward said emissive electrodve, means to'maintain said photoemissive electrode andsaid electron beam source at substantially tlie-samepotential to decelerate said beam in the absence of light onsaid emissive electrode to substantially zero velocity adjacent said emissive electrode, means to subject said emissive electrode to a fluctuating source of light to develop television'signals, a phototube` responsive to abnormal intensities of light from said source to develop a voltage impulse substan-Y tially simultaneouslyv with and during the occurrence of abnormal light intensities from said source, and means comprising an amplifier connected between said phototube and said c ollecting electrode to utilize said developed voltage impulse to, reduce the positive potential applied to said collecting electrode and interrupt the passage of said beam to said emissive electrode and the collection of photo-electrons from said emissive electrode during the pccurrence of said abnormal light intensities from said source.

6. A television transmitting system as claimed in claim 5 including means to Ydevelop a second voltage impulse of longer duration than said first-mentioned Voltage impulse and means to apply said jsecond'voltage impulse to said beam control electrode to increase the intensity of said beam'immediately following the interruption of the passage of said beam to said emissive electrode andthe collectioniof photo-electrons therefrom. A l

'7. In a television transmitting system including a tube having a'photo-emissive electrode, an electron collecting electrode to collect photo-electrons from Ysaid emissive electrodean electron beam source, an electron beam control electrode, means 'to normally maintain said control electrode at a potential with respect to said cathode to develope abeaml ofjelectrons, `means to normally maintain said collecting electrode at a positive potential with respect to said electron Y beam source to direct the electron beam toward said emissive electrode, means to maintain said photo-emissive electrode and'said electronbeam source at substantially the Same potential to decelerate said beam in the absence of light on said emissive electrode to substantially zero velocity adjacent said emissive electrode, means to subject said emissive electrode to a source of light to develop television signals, means responsive to abnormal intensities of light from saidsource to develop a negative voltage impulse substantially simultaneously with the occurrence of the abnormal intensities from said source, means to derive a time' delayed positive voltage impulse from said developed.negativev voltagerimpulse, means to apply said negative voltageA impulseA to said collecting electrode; to, interrupt the passage of said beam to said emissive electrode andthe col-'- lection of photo-electrons from said emissive electrode, and means to apply said `delayed positive voltage impulse to said beam control electrode to intensify said electron beam immediately following the application of said negative voltage impulse to said collecting electrode.

8. In a television transmitting system a tube of the low velocity beam scanning type including a cathode to develop a low velocity electron beam, a beam control electrode, a photo-emissive target adapted to be scanned by said beam and an electrode intermediate said cathode and said target to collect photo-electrons liberated from said target in response to light thereon, means to maintain said electrode at a positive potential with respect to said cathode, means to focus light from a light source subject to sudden high intensity light variations on said target to develop television signals representative of the normal light intensity of said source, a light sensitive device comprising a photocathode and an output electrode, means to focus light from said light source on said photocathode to develop electrical impulses, means to amplify and limit said electrical impulses with respect to predetermined-intensities of said sudden high intensity light variations from said source and derive limited potential impulses negative with respect to said cathode, and means to apply said limited negative potential impulses to said collecting electrode to render said tube inoperative during the pre- Y determined high intensity periods of said high intensity light variations of said source.

9. A television transmitting system as claimed in claim 8 including means to develop additional voltage impulses which are positive'with respect to said cathode from said first-mentioned developed electrical impulses, means to apply said positive voltage impulses to said beam control electrode to increase the intensity of the 'electron'beam from said cathode, and means to delay the development of said positive. voltage impulses whereby the impulses are eilective in increasing the intensity of said beam after the application of said limited negative impulses to said collecting electrode. Y

OTTO H. SCHADE. 

